The present invention relates to a permanent-magnet-excited electric motor having a first active part which is fitted with permanent magnets, and a second active part which can be moved relative to the first active part by means of magnetic force of the permanent magnets. The present invention further relates to a method for operating a permanent-magnet-excited electric motor of the aforesaid type.
The torque of electric motors typically decreases as rotational speed increases. This is illustrated by way of example in the diagram according to FIG. 1. Characteristic curve 1 represents a permanent magnet (PM) synchronous motor having strong permanent magnets. Their residual induction Br equals 1.1 T. At low rotational speeds the synchronous motor possesses a torque of up to 25 Nm and more. The torque decreases rapidly with increasing rotational speed and reaches the value 0 Nm at around 4500 min−1. This means that said synchronous machine possesses high torque at low rotational speeds, yet only low output power at high rotational speeds.
Characteristic curve 2 in FIG. 1 represents a PM synchronous motor having weaker permanent magnets. At low rotational speeds the torque reaches almost 20 Nm and then drops to 0 Nm at approximately 6200 min−1. In the case of these permanent magnets the residual induction Br amounts to 0.8 T. Thus, although the synchronous machine has a lower torque at low rotational speeds, it already has a higher output power at higher rotational speeds.
Characteristic curve 3 likewise represents a PM synchronous motor having relatively weak permanent magnets. in this case the residual induction Br equals 0.4 T. At low rotational speeds the torque attains the value 10 Nm. The torque decreases only slowly and at 8000 min−1 still has a value of approximately 5 Nm. This in turn means that at low rotational speeds the PM synchronous motor possesses a low torque, yet at high rotational speeds it can in contrast deliver very high output power.
The permanent magnets of a synchronous machine should therefore be chosen according to their desired operating point (rotational speed). If a high torque at low rotational speeds is required, then strong permanent magnets need to be employed. If, on the other hand, high output power at high rotational speeds is required, then weaker permanent magnets should be used for preference.
If it is aimed to achieve a high torque at low rotational speed and high output power at high rotational speeds, then use is made of the field weakening effect of the permanent magnets in PM machines. This is achieved by impressing a direct-axis current component whose field overlays the field of the permanent magnets and thereby weakens said field. A disadvantage thereof is on the one hand that for its part said field-weakening current generates losses in the windings. On the other hand the impressed current must not be allowed to become too strong so that the permanent magnets are not irreversibly weakened. Furthermore harmonic fields are produced due to saturation, which again increase the iron losses.